Oil may be harming base of food web, early results suggest.

Weeks after the U.S. government claimed that the "vast majority" of oil from the Gulf of Mexico spill has been taken care of, oil has possibly been found deep on the Gulf seafloor, scientists announced this week.

What's more, tiny deep-sea creatures are showing a "strong toxic response" to hydrocarbons, an ingredient of oil, according to preliminary results released Tuesday by the University of South Florida (USF).

During a ten-day research cruise in August, the USF team sampled water and sediments in DeSoto Canyon—a nutrient-rich gully east of the Deepwater Horizon wellhead that's also the birthplace of many commercial fish species.

By shining ultraviolet light on the samples, the team indirectly detected hydrocarbons in the sample that seem to have the same fluorescent fingerprints as oil from the wellhead, which was capped July 15. (Related pictures: "Glowing Oil Could Aid Gulf Spill Cleanup.")

The early results are reminders that the oil hasn't been fully dispersed or degraded away, said cruise member David Hollander, a USF chemical oceanographer.

"Don't let your finger off the pulse of this," he said. But Hollander emphasized that his team's observations are preliminary: "I hope they don't get misconstrued as scientific fact."

Robert Carney, a biological oceanographer at Louisiana State University (LSU) in Baton Rouge, also cautioned that "we have to be exceptionally careful" about interpreting the results.

"We cannot base conclusions on short cruises with small groups of people—it has to be a [major] scientific undertaking," Carney said.

Gulf Seabed "Glow" Caused by Oil?

To collect their seabed samples, the USF team used a box corer, a geological tool for scooping up sediments that's "as primitive as it gets," Hollander said.

The samples not only emitted the telling fluorescence, but their glow persisted even after the samples had been frozen for seven hours, suggesting that living things weren't causing the light, Hollander said.

But there could still be other substances responsible for the brightness, such as fluorescent minerals. Further laboratory tests—expected early next week—will confirm whether the samples contain Gulf oil, he said.

But he added that oil also occurs naturally in the Gulf. Hydrocarbons, for instance, often escape from natural oil seeps on the seabed. (Read more about the Gulf of Mexico's natural seeps.)

Oil Means Lights Out for Tiny Creatures

The USF team also tested how bacteria and phytoplankton—microscopic marine plants—were responding to potentially oil-contaminated water from both the Gulf's surface and the deep ocean.

"There's been a lot of attention toward upper tropic levels—the sharks, the birds, the turtles," Hollander said. "We're asking what's going on at the base of the food web. To our surprise these things haven't come out either through [government] agencies or the scientific press—and indeed it appears there is a response."

Healthy bacteria and phytoplankton give off a gentle glow of visible light. So the team took otherwise fit organisms and exposed them to water samples from the surface and from the deep sea to see if that diminished their glow.

Phytoplankton showed an "enormous reduction" in light when they were exposed to deep-sea samples, but the plants seemed unaffected in the surface samples, Hollander said. The results were the opposite for bacteria.

The researchers aren't yet sure why the two types of organisms reacted differently. It's possible, for example, that different species have different reactions to the toxic effects of oil and chemical dispersants, Hollander said.

One theory is that, in general, phytoplankton are more sensitive to dispersants—which may be more plentiful in the deep sea—while bacteria (not the oil-eating variety) react poorly to the purer oil still on the water's surface.

Hollander and colleagues next plan to test how larval fish and tiny animals called zooplankton respond to the samples.

Deep Gulf Oil "Not Good News"

For Texas Tech University ecotoxicologist Ron Kendall, the bacteria and plankton's responses confirm his initial fears.